fix: Handle custom ObjectReaders lacking createInstance in JSONObject…

[jujn]

….to, for issue #3901

对没有实现 createInstance 方法的类型使用 JSON.parseObject(this.toJSONString(), clazz, features)，但这里会增加一次序列化的开销，目前没有想到更好的修复方案

[Copilot]

Pull request overview

This PR fixes issue #3901 by adding fallback handling for custom ObjectReader implementations that don't implement the createInstance method. When a custom reader throws UnsupportedOperationException during object instantiation, the code now falls back to serializing the JSONObject to a JSON string and re-parsing it, which invokes the custom reader's readObject method instead.

Key Changes:

• Added try-catch block to handle UnsupportedOperationException in JSONObject.to(Class<T>, JSONReader.Feature...)
• Fallback uses JSON.parseObject(this.toJSONString(), clazz, features) when createInstance is unsupported
• Added test case to verify the fix works with a custom ObjectReader that only implements readObject

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
core/src/main/java/com/alibaba/fastjson2/JSONObject.java	Added exception handling in the to() method to catch UnsupportedOperationException from createInstance() and fall back to JSON string parsing
core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3901.java	Added test case demonstrating a custom ObjectReader that only implements readObject and verifies the fallback mechanism works correctly

Comments suppressed due to low confidence (1)

core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3901.java:34

• UserReader should be made static, since the enclosing instance is not used.

    public class UserReader implements ObjectReader {

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[jfinal]

这里的问题在于即便正确实现 createInstance 方法，仍然会出现另一个新的异常。 在 catch 中通过 JSON.parseObject(this.toJSONString(), clazz, features) 的方式一是抛出异常有性能损耗，二是多一次 toJSONString() 转换有性能损耗。

更希望是在实现 createInstance 方法的基础之上，能解决那另一个异常，这样起码性能上有保障。

[jujn]

@wenshao ，简单总结该问题就是，用户注册自定义反序列化器，仅实现了 readObject ，但是如果调用 JSONObject/JSONArray 相关 api 时，会调用 ObjectReader 默认的 createInstance （map、collection 版本）直接抛异常。我发现框架没有办法预判用户期望的反序列化行为，所以为保持高性能貌似只能让用户自定义 createInstance 。直接在文档里注明可以吗？（可以参考我新提交的代码示例）